We explain what a pulley is and what is the history of this machine. In addition, the types of pulleys that exist and the parts that compose it.
A pulley transmits force and acts as a traction mechanism.
What is a pulley?
A simple machine designed to transmit force and operate as a traction mechanism is known as a pulley, reducing the amount of force necessary to move or suspend in the air a weight. It consists of a wheel that rotates on a central axis and provided with a channel on its periphery through which a rope passes.
The pulley can also be defined as the fulcrum of a rope that moves around it without giving it a complete turn; such is the definition of the French scientist Hatón de la Goupillière. Thus, at one of the ends of said rope a endurance or weight, while in the other one power or strength.
The use of pulleys is very common in the areas of construction, loading or unloading of vehicles and many others, in which a system is required to mobilize large weights with considerably less force.
For example, the mechanism designed to extract Water of a deep well, so common in movies and medieval imaginary, consists of a bucket tied to a rope that passes through a pulley. Thus, by pulling the free end, the bucket full of water (and considerably heavy) can be raised to the edge of the well.
Not much is known about the invention of the pulley. The only note about it in the literature history accuses Archimedes as its inventor, although he may well have been only a scholar and enthusiast of its use.
Plutarch tells in his bookParallel lives (100 BC) that the Greek Archimedes affirmed the King of Syracuse, Hiero, to whom he attached a bond of friendship, that given a force and a point of support, any weight could move, even that of a Earth whole. His friend demanded a practical demonstration: he packed a royal navy ship with cargo and passengers and asked the philosopher to move it to a dry dock.
After designing the proper pulley system, Archimedes sat at a distance and pulled almost effortlessly on a rope, causing the ship to rise and move so steadily that it seemed to still remain in the water.
Depending on the number of pulleys, they can be single or combined.
There are two ways to classify pulleys:
- According to yourdisplacement. One can speak of pulleysfixed when they are suspended from a fixed point; ormobiles when it comes to a set of two pulleys: one fixed and the other mobile.
- According to your number. Depending on whether it is a pulley acting alone or an interconnected set of them, we can speak of pulleys simple or pulleys combined orcompound, respectively.
Parts of a pulley
Every pulley is made up of four fundamental parts:
- Axis. The fixed portion around which the pulley is inserted or suspended and which allows its free rotation. It is the immobile and central part.
- Tire. The outer zone of the pulley, where the throat is located where the rope passes.
- Body. The middle part of the pulley, between the hub and the rim, designed to rotate under the action of force, provided with arms or nerves to facilitate its movement.
- Cube. The inner part of the pulley (the cylindrical hole that the shaft attaches to).